This Is Why Icelandic Eruptions Can Be So Problematic For Air Travel

The most dramatic images of volcanoes often feature those tall, unnerving eruption columns soaring up into the day or night sky, shimmering with flecks of cooling lava and booming and roaring across the land. When they aren’t a direct threat to humans, they truly are a sight to behold – a demonstration of nature’s most infamous crucible.

Leaving out pyroclastic flows of this particular conversation – about which you can read here – you’re probably aware the Iceland’s eruption columns can prove troublesome for air travel. The eruption of Eyjafjallajökull back in 2010 famously caused the most expansive shutdown of European airspace since the close of the Second World War.

Volcanic ash is the result of the sudden cooling of fine particles of lava, thrown out of the vent explosively, fragmented into incredibly tiny pieces, and exposed to the elements, be that water or air, or both.

Ash billows from a volcano in Eyjafjallajokull on April 16, 2010. (HALLDOR KOLBEINS/AFP/Getty Images)

Give this primarily glassy substance enough heat, though, and it remelts into that sticky, hot lava. As it so happens, the jet-engines of planes are incredibly hot too – hot enough to trigger the melting of volcanic ash into lava, which can ultimately destroy the engine and bring the plane down, as has happened several times in the past. Fortunately, ash detection methods are far more advanced these days, and this type of transportation accident is essentially unheard of.

Someone recently asked me about Iceland’s volcanic eruptions; specifically, they were curious as to why the 2010 event there caused so much trouble. Yes, when volcanoes erupt explosively enough, and there’s an eruption column drifting up into the sky, it is always danger no matter where in the world it happens, and planes are quickly rerouted.

Still, why did Iceland’s Eyjafjallajökull cause so much trouble back then, and why even today are its other volcanoes often causing similar worries among airlines and volcanologists? They aren’t usually as explosive as the types of eruptions produced by pointy, mountain-esque stratovolcanoes like Mount St. Helens, and normally you need a heck of a lot of energy to sustain a gigantic eruption column – so why is Iceland so, well, peculiar?

The aviation zones shut down (red) or restricted (orange) by the 2010 eruption in Iceland.DeltaFalcon/Wikimedia Commons; Public Domain

First, let’s take a look at how you generate an eruption column in the first place. There are a few different types, but the “classic” eruption column has three different sections.

Normally, to get a column, you need the volcano to be erupting fairly energetically. This is a deeply complex process, and one that isn’t still fully understood, but the basics are that you need a very gloopy (viscous) magma, a lot of dissolved volatiles, and a persistent source of heat to keep the magma within the chamber and its various passages – the sills, dykes, conduits and so on – mobile and eruptible.

This magma is kept under a high confining pressure by the surrounding rock. If the internal pressure of the magma overcomes the rock, and the overlying roof cracks, magma will rush upwards. As it rises, and the confining pressure lessens, the magma decompresses, allowing the dissolved volatiles like water, carbon dioxide and sulphur compounds, to exsolved as bubbles of gas. This, in turn, lowers the density even further.

Generally speaking, the steeper the gradient – or the greater the difference – between the pressure of the magma and the atmospheric pressure, the more explosive the eruption will be when a critical point is reached.

The sun sets in a sky dusted with ash on May 5, 2010 as the volcano volcano continues spewing ash. (HALLDOR KOLBEINS/AFP/Getty Images)

This kickstarts the first phase of the complete, tripartite eruption column: the gas thrust region. This section is propelled by the explosive decompression itself, and begins wherever in the conduit – the central “windpipe” of the volcano, if you will – the magma is violently fragmenting.

If there’s enough energy to push this column up high enough into the sky, you’ll get a convective region, one in which is sustained in several ways. Unlike the steady flow of the gas thrust region, here, colder air gets mixed into a warm, turbulent cloud. If too much cold air gets mixed in, the eruption column gets too dense and collapses.

As the kinetic energy from the explosions below runs low, the small flecks and blebs of lava will begin to cool. This will release heat, which warms the incoming air, expand it, reduce its density, and keeps things buoyant. If you’ve got enough lava cooling, the column will remain moving upward.

Then, right at the top, you’ve got the beautifully named umbrella region. At a certain height, the eruption column’s density will match that of the increasingly thin, low-pressure atmospheric environment. Carried by whatever upward momentum it has left before it loses energy, the ashy marvel will billow up and outwards. If the ash column is particularly high – breaching the stratosphere, which starts about 18 kilometers up – powerful streams of air can carry this ash around the region or even the planet, with potentially dramatic consequences.

Depending on a wide range of factors, eruption columns can be anywhere from a few hundred meters high to a tens of kilometers. They can be sustained for days, weeks or months, or just for a few hours or barely that. When the column loses its explosive energy at the base, or simply succumbs to the cool air around it, it falls into an angry oblivion.

So – now that you’re well versed with the science of eruption columns, why was Iceland’s 2010 outburst so problematic?

After the eruption started properly on 14th April – after a smaller initial event on the 20th March – it lasted for 39 days, and despite being described as a “modest” event, one that wasn’t that explosive at all, it created a persistent column of ash. Although about half of the ash fell on Icelandic soil, most of the rest spread out over a distance of 7 million square kilometers over Europe and the North Atlantic Ocean, with some reaching continental Europe.

One landmark study detailing the event explains that the amount of magma unleashed was nowhere near that of more explosive eruptions, but it had a sustained eruption column that somewhat resembled them. Similarly, the wide ash dispersal is more commonly found in more violent paroxysms. For one thing, the eruption column never really had the features of a classic column - it looked more like a wonky fountain of ash, with a warped umbrella. So how did such a comparatively weak eruption with a weird eruption column spread ash all over European airspace?

Much of the pervasiveness and problem with the eruption column back then came down to its slightly unusual eruption style. For a few days, thanks to its viscous silicic magma source explosively depressurizing, it was sustained explosively, with a powerful gas thrust region, and the column reached a maximum height of 10 kilometers. Not stratospheric heights, but around the cruising altitude of passenger jets. From that point onwards, it alternated between an effusive, lava flow-emitting phase and more explosive phases.

Research has shown that the eruption should have ended sooner than it did; however, the second explosive phase took place because of a fresh injection of incredibly hot basaltic magma into the underlying chamber. This added fresh heat and yet more volatiles, ensuring the magma remained molten, mobile and buoyant for longer than expected. This kept the ash column going for longer than one would expect, causing periodic shutdowns of airspace after the near-blanket ban in April was eventually revoked.

One additional major factor was how fine the ash was; the smaller the ash, the longer it can drift up in the lower atmosphere. Generally, ash is finer when the initial fragmentation is more explosive, and in this case, the ice layer likely played a role here.

If you erupt lava on to ice, very little happens. Some steam rises, the ice largely remains intact, and the lava solidifies. This is partly thanks to something called the Leidenfrost effect: the contact of the lava and the ice produces a layer of vapour, which insulates the ice/water from the lava for some time. Eventually, the vapour film collapses, the ice/water makes direct contact with the lava, and it boils away quite vigorously, releasing gas quite rapidly.

Erupt it into ice or water in a high pressure environment, however, and something rather remarkable happens. The Leidenfrost effect still applies, but the collapse of the vapor film happens far more violently. When magma or lava is turbulently wrapped up in a coolant, and multiple vapor films in a magma/water mix occur simultaneously, a significant about of thermal energy is transferred to the water in the blink of an eye. As you'd expect, this can create a rather remarkable series of explosions.

If lava is released rather than just steam, this is known as a phreatomagmatic eruption, the type that almost killed a BBC film crew on Mount Etna earlier this year.

Those are the basics, but the key point here is that phreatomagmatic blasts within a volcano’s throat can create pronounced fragmentation. This likely contributed to the extremely high proportion of very fine ash coughed out of Eyjafjallajökull back in 2010, which escaped over the sea through the eruption column’s umbrella region.

The ash itself was fragmented so powerfully that it even its shape was unusual for this type of eruption. It wasn't exactly aerodynamic, which allowed it to stay up in the sky for longer.

Part of the problem also happened to be the persistent northwesterly winds. If there was a different direction to the wind, or it wasn’t windy at all, Europe would have been spared from the aviation shutdown. At the same time, the atmospheric circulation above Iceland and Northern Europe wasn’t as it usually was, which also contributed to the ash dispersal.

So, in sum, despite being a modest eruption, this volcano had an odd, regenerating gas thrust region, and an umbrella segment filled with particularly fine ash. This conspired to cause Europe a major headache back then – which surprised everyone, including many volcanologists. Indeed, that’s why Iceland in general is such a pain for aviation over Europe: it’s position near to major flight paths, its geology, its glorious covering of ice, and its plethora of volcanoes make for one hell of a mix.

An aerial picture taken on September 14, 2014 shows lava flowing out of the Bardarbunga volcano in southeast Iceland. (BERNARD MERIC/AFP/Getty Images)

Icelandic eruptions don't always cause a total shutdown of European airspace. In 2011, an eruption at a different volcano was nowhere near as consequential. Still, as 2010 revealed so dramatically, volcanologists should now plan to expect the once-unexpected.

Apologies if you were expecting a simpler answer. Science, as it so happens, is complicated. It’s also the most remarkable storytelling device ever developed by humans, because, as has been hopefully made clear here, it can unravel volcanic eruptions, piece by piece.

Robin George Andrews is a doctor of experimental volcanology-turned-science journalist. He tends to write about the most extravagant of scientific tales, from eruptions and hurricanes to climate change and diamond-rich meteorites from destroyed alien worlds - but he's always...